The core idea of MIMO technology is that multiple antennae are used to transmit and receive multiple signals in order to fully utilize space resources. With respect to narrowband channels, the channel capacity of a MIMO system is nearly in direct proportion to the minimum number of receiving and transmitting antennae, and the channel capacity and spectrum utilization rate can thus be increased dramatically without increasing transmission power of the antennae and spectrum resources.
The Orthogonal Frequency Division Multiplexing (OFDM) technology is a multi-carrier transmission technology, which divides a channel into multiple orthogonal sub channels, converts high-speed data stream into parallel low-speed sub streams and modulates the parallel low-speed sub streams to be transmitted on the sub channels. The typical advantages of an OFDM system include low InterChannel Interference (ICI) among the sub channels, high utilization rate of spectrum resources and relatively strong resistance to both frequency-selective fading and intersymbol interference.
In order to fully exploit the advantages of MIMO technology and OFDM technology, the space resources are properly utilized based on the OFDM technology by combining the MIMO and OFDM technologies to form a MIMO-OFDM system. In the MIMO-OFDM system, higher data transmission rate can be achieved while the Quality of Service (QoS) is guaranteed.
Cyclic delay is usually introduced into the data to be processed in the MIMO-OFDM system to obtain the performance of frequency-selective on flat channels. In other words, the data to be transmitted via different antennae correspond to different cyclic delays to obtain Cyclic Delay Diversity (CDD). Introducing cyclic delays is called CDD processing. Furthermore, in order to improve the signal transmission quality in the MIMO-OFDM system, the data to be processed is usually pre-coded by using a preset code book, i.e. by multiplying the data to be processed by the code word that matches the current channel most in the code book. The CDD processing and the precoding are often combined into a CDD preceding technique to relieve the problem of performance deterioration of multi-user scheduling on flat channels and thus improve signal quality.
FIG. 1 is a flow chart in which data are processed by using a conventional CDD precoding technique on the base station side. As shown in FIG. 1, the method of processing the data, e.g. on the downlink, is described below.
In Block 101, radio resources are allocated to user terminals and the signal flow to the user terminals forms the data to be processed.
Because the data to be processed is on the downlink, the base station allocates the radio resources to the user terminals in the MIMO-OFDM system in order to ensure normal data transmission of multiple users. Furthermore, because the base station needs to communicate with multiple users on the downlink, the signal flows corresponding to different user terminals respectively are combined to form the data to be processed. For example, the signal flow corresponding to one user is put into one row of a matrix of the data to be processed.
In Block 102, a code word corresponding to the user terminals in the cell is chosen from a preset code book to precode the data to be processed.
The same code book which includes a plurality of code words for preceding is respectively set in the base station and the terminals in the MIMO-OFDM system. A terminal measures and determines the current channel quality, chooses a code word that best matches the determined channel response from the code book and reports the index of the chosen code word to the base station. Upon receiving the code word index from the terminal, the base station finds the code word corresponding to the code word index in the code book and sets the found code word as the code word corresponding to all user terminals in the cell where the reporting terminal is located. After the code word corresponding to all user terminals in the cell is determined for the data to be processed, the signal flows of the user terminals among the data to be processed are multiplied by the found code word to obtain a precoding result.
In Block 103, the precoding result is processed by the CDD processing based on preset cyclic delays of the antennae.
Generally, the OFDM chip serves as the basic unit of the cyclic delays of the antennae, and the cyclic delays are varying slowly over the long term. For example, in a two-input-two-output MIMO-OFDM system, both the transmitting antenna 1 and transmitting antenna 2 would transmit signals [S1, S2, . . . SN], and the transmission of S1 is earlier than that of SN. If the cyclic delay corresponding to transmitting antenna 1 is −1 OFDM chip and the cyclic delay corresponding to transmitting antenna 2 is 2 OFDM chips, the signals transmitted by the transmitting antenna 1 is SN→S1→ . . . SN-2→SN-1 in chronological order, and the signals transmitted by the transmitting antenna 2 is S3→ . . . →SN→S1→S2 in chronological order. As can be seen, the CDD processing applied to the signals actually means that n symbols chronologically behind in the sequence are transmitted early in the sequence if the cyclic delay is smaller than 0, and n symbols chronologically ahead in the sequence are transmitted later if the cyclic delay is greater than 0, where n is the absolute value of a cyclic delay.
The Blocks 102 and 103 can be combined into one Block, in which the CDD precoding is realized by multiplying the signals to be transmitted by the code word of CDD precoding. The code word of CDD precoding can be expressed as WCDD(k)=diag(e−j2πkd1/NC, e−j2πkd2/NC, . . . , e−j2πkdNt/NC)WConventional, where WCDD(k) indicates the code word of CDD preceding, Wconventional indicates conventional code word of preceding, and the diagonal matrix (e−j2πkd1/NC, e−j2πkd2/NC) is used for the CDD processing, k indicates the serial number of a sub-carrier, Nc indicates the number of sub-carriers and di indicates the cyclic delay of No.i antenna.
Thus, the conventional process of data processing is terminated.
After the above process, Serial/Parallel (S/P) transform, Inverse Fast Fourier Transform (IFFT), the insertion of cyclic prefix and the like can be applied to the CDD processing result to obtain the signals to be transmitted and then transmit the signals over corresponding antennae.
As can be seen from the above data processing, the user terminals in the same cell use the same precoding code word, i.e. use the same WConventional. However, there may be user terminals of different demands in one cell, and these user terminals may have different requirements for a precoding code word which reflects the channel response. However, one code word can satisfy only the user terminals of one certain requirement, and would affect the signal transmission quality of other user terminals. For example, a code word which can achieve low bit error rate would not satisfy the user terminals requiring high frequency spectrum. Moreover, the existing cyclic delay varies only slowly in a long term, and the existing cyclic delay is therefore unable to fit the channel variation speed of the user terminals moving at a higher speed in the cell. As a result, the signal transmission quality of the user terminals moving at a high speed cannot be guaranteed. It can thus be concluded that the conventional method of data processing does not support choosing a CDD precoding code word according to different requirements of user terminals and thus can not satisfy the demands of all user terminals, therefore affecting the signal quality in the MIMO system.